SUMMARY: The effects of testosterone on weight changes, incorporation of labelled precursors into RNA and proteins, and on ultrastructure of the levator ani muscle of male rats in which castration or denervation was performed, were studied. No increase of weight occurred in the denervated levator ani muscle after castration. There was no increase in the incorporation of labelled precursors into RNA and proteins in vitro in mature rats, if the muscle was denervated. However, incorporation still increased when denervation was performed in sexually immature animals. The levator ani muscle, which normally undergoes postnatal involution in the female rat, could be maintained even after postnatal denervation if testosterone was administered at birth, the hormone acting as a nerve-independent trophic agent. The increase of protein synthesis in the levator ani of mature, male, castrated animals, induced by testosterone administration, was abolished after denervation of the muscle. The activating influence of testosterone upon protein synthesis was, however, still present in immature male animals.